1. Field of the Invention
The invention relates to a method and process for generating useful power. More particularly, the invention provides a combined reheat gas turbine and steam turbine cycle wherein a reheat combustor accepts a heated and compressed gas produced by a conventional gas generator, adds fuel and delivers reheated gas to a power turbine for directly generating power, exhaust gas from the power turbine forming superheated steam to drive the steam turbine. Alternatively, the reheat combustor for reheating the gas generator exhaust gas incorporates heat exchange means for superheating steam therein prior to delivery of reheated gas to the power turbine and optionally for production of reheated turbine steam for return to the steam turbine to furnish additional power thereto.
2. Description of the Prior Art
Research and development is currently being directed toward many configurations of power systems involving gas turbines because of the growing awareness of impending world energy shortages. The present invention relates to the need to focus technical attention to the reheat cycle and with use of the apparatus and processes of the present invention, the reheat gas turbine cycle and combined gas reheat and steam reheat cycle can appreciably increase power plant thermal efficiency to approximately an over-all 50% efficiency level or higher.
The reheat gas turbine cycle itself is well-known and has received considerable attention over the years, particularly in Europe. A regenerator has been used to heat a compressor's discharge air to improve cycle efficiency, and intercooling has been suggested for the same purpose. It is known that reheating can increase power output by 35 to 40%, but without use of regeneration, over-all cycle efficiency is degraded.
Another example of a reheat gas turbine cycle presently employed is the afterburner of a jet engine for aircraft use, such as in military aircraft and in certain commercial supersonic planes. The jet reheat cycle has been developed to get reliable service in applications where augmented power output is required for a short or limited time. The greatly increased power output comes at the expense, however, of markedly increased fuel consumption, owing to which, commercial applications have gone to efficient high bypass ratio fan jets for subsonic flight. Technology developed therefrom has made available second generation high-ratio high-firing temperature gas generators for industrial applications.
Yet, despite the existence of known technology, combined cycle power plants, such as those which utilize the processes and apparatus of the present invention, have not been developed. Attention has not been given to reheat gas turbine combined steam turbine cycle power plants because it has been thought that the degradation in efficiency of the reheat cycle would not offer an advantage, and also that increased fuel consumption would result. However, as will be pointed out hereinafter, thought must be given to the existence of higher level heat available in the exhaust and the over-all entropy changes as well as the concept of extracting maximum work at the high working fluid temperature levels possible with the present invention to obtain highest efficiency. Another explanation of the failure of others to utilize a reheat cycle process is the feeling that greater complication and cost of controls, and additional burner, compatibility of nozzle area, start-up procedures, and the like, would offset any advantage in specific power output, particularly at a higher fuel consumption.
Yet another reason can be cited for the failure of others to point in the direction of present invention. Aircraft derived gas turbines offer an advantage in potential physical arrangement for the reheat cycle in that a reheat combustor can be readily added between the gas generator and the power turbine, whereas conventional industrial or heavy duty gas turbines are ordinarily single shaft units for power generation, such conventional units not readily lending themselves to addition of a reheat burner. Moreover, second generation aircraft gas turbines fire at elevated temperatures and utilize high compression ratios suitable for reheat cycles, while industrial units, because of a single shaft configuration, are limited to lower ratios.
With the appearance of an increasing number of high-temperature and high-pressure-ratio gas turbines which lend themselves to reheat cycles, use of such equipment according to the process and teachings of the present invention becomes technically feasible, affording a practical utility for the generation of useful power, such as electrical power.